1. Field of the Invention
This invention relates to an improved method for effecting catalytic reactions in fixed catalyst beds with mixed-phase co-current flow. More particularly, this invention is directed to a method of improving flow distribution of reactants through a fixed bed of catalytic material.
2. Description of Prior Art
Many chemical, petroleum processing and petrochemical reactions require effective contact of the feed with a catalyst used in the process. One of the means for effecting such a contact comprises the use of a fixed catalyst bed reactor wherein the feed is passed through the catalyst bed. If more than one phase for the reactant is needed to carry out the reaction, the reactants may be passed either co-currently or counter-currently through the catalyst bed. Whether the reaction is conducted co-currently or counter-currently, it is important to assure that the feed is distributed substantially uniformly throughout the fixed bed of catalyst. Otherwise, the likelihood of channeling of the reactants through the catalyst bed is increased, which results in reduced conversion, non-uniform product composition, increased recycle rates, short cycle length and loss of control over the course of the reaction.
Thus, attempts have been made in prior art to improve distribution of the reactants throughout the catalyst bed. For example, Scott, U.S. Pat. No. 4,138,327, discloses a vapor liquid distributor for catalytic reaction beds, comprising three annular form, catalyst-free volumes placed within the catalyst bed and defined by the interior surface of the catalyst chamber and three cylindrical walls in concentric relationship therewith. U.S. Pat. No. 3,146,189 discloses a device for the initial distribution of a vapor/liquid feedstream to a fixed bed of catalyst particles. The liquid and vapor components of the feed are introduced separately into the catalyst bed through a horizontal plate containing liquid and vapor downcomers. The vapor downcomers extend into the bed of catalyst particles, and they are designed to afford lateral vapor flow through the bed of catalyst.
Effron et al., U.S. Pat. No. 3,524,731 discloses a distributor apparatus in a reactor designed to minimize channeling effects of the mixed-phase components used therein. The distributor comprises a horizontal plate having inserted through it a plurality of short tubes and longer tubes which are notched near the point of contact of the tubes with the horizontal plate.
U.S. Pat. No. 3,685,971 discloses a distributor similar to that of U.S. Pat. No. 3,146,189. The distributor of the U.S. Pat. No. 3,685,971 is located adjacent to the reactant inlet port, and its discharge end comprises a plurality of depending, spaced-apart concentric baffle members which produce an outward deflection of concentric, annular-form flow streams to an area of the lower catalyst bed.
Ballard et al., U.S. Pat. No. 3,218,249, disclose a bubble cap feed arrangement which improves the distribution of mixed vapor-liquid reactant phases over a fixed bed of catalyst.
Kassarjian, U.S. Pat. No. 3,732,078, provides a different means for minimizing channeling in a fixed catalyst reactor bed. The redistributor of Kassarjian comprises a deflector, e.g., a six inch wide rim around the inside perimeter of the reactor, in conjunction with a load of low flow resistance particles, e.g., balls, loaded immediately below the rim.
Trasi and Khang demonstrated that for gas-liquid counter-current flow in a packed bed with intermittent voids along the axis significant improvement in the distribution of the reactants was observed (Industrial and Engineering Chemistry Fundamentals, Vol. 18, No. 3, pages 256-260, 1979).
However, none of the aforementioned publications has successfully solved the problem of non-uniform distribution of co-current flow of liquid and liquids/vapor reactants through a packed bed of catalyst. Moreover, the problem of non-uniform distribution of the reactants through a packed catalyst bed in relatively low liquid velocity systems (liquid hourly space velocity of 2 or less), e.g., in the processing of heavy natural and synthetic crudes, has not been heretofore adequately addressed. At such low liquid flow rates there is a tendency for the liquid reactant to reach its liquid equilibrium distribution between the wall of the reactor and the liquid trickling over the catalyst surface because, it is thought, greater void volume of the catalyst is found in that region. Consequently, a uniform liquid distribution attained near the top of the packed catalyst bed does not guarantee a uniform liquid distribution a few feet down the bed. The non-uniform distribution occurs primarily because of the increased rate of liquid flow near the wall region of the reactor, wherein the void volume of the catalyst is usually greater than in the center of the usually circular cross-section of the catalyst bed.